The introduction of the synchronous digital hierarchy SDH by the CCITT (Comite Consultatif International Telephonique et Telegraphique) permits the implementation of a worldwide uniform transmission network.
The bit rates of the SDH are specified in CCITT Recommendation G.707. The signal structure at the network node interface is contained in CCITT Recommendation G.708, and the multiplexing structure in CCITT Recommendation G.709.
An SDH frame according to CCITT G.708 consists of octet-oriented synchronous transport modules STM-1 with a bit rate of 155,520 kbits/s.
The frame consists of 270 columns containing 9 rows each. 9.times.9 octets are dedicated to a so-called section overhead SOH, and 261.times.9 octets to a useful-information part, called "payload".
Row 1 of the section overhead SOH contains a frame sync word with a Length of 6 octets. Row 4 of the section overhead contains a pointer AU-4 PTR which addresses a path overhead POH of a virtual container VC4 of the payload. The path overhead POH contains an additional pointer H4 which addresses cell headers of a 260.times.9-octet cell-structured container C4. The cell structure of the container allows the asynchronous transmission of data in the form of ATM cells (asynchronous transport modules) within a synchronous frame (SDH).
To establish fast frame synchronization, it is known to continuously scan n-bit bit patterns of the received bit stream at a local clock rate, namely at 1-bit intervals, with the aid of a predetermined n-bit frame sync word, to compare the n-bit bit patterns with the frame sync word, and to output frame sync information in case of agreement (see published German Patent Specification 28 55 676 A1). Before it can be inferred with a high-degree of reliability that the transmission link is synchronized, the periodically occurring sync word must be detected repeatedly at predetermined intervals. This has the disadvantage that it takes very long, at least two frame lenghts, until synchronization is achieved.
In systems where ATM cells have to be transmitted, the synchronization time should be very short since service quality is greatly impaired by synchronization losses.
It is also known to transmit cells of constant length without external frames (asynchronous transport modules).
In case of purely cell-structured transmission, cells of equal length are transmitted with a periodicity of 53 octets, each cell containing 5 octets for a cell header and 48 octets for the payload. The cell header is encoded in a polynomial code, so that the uncoded payload can be distingushed from the cell header by decoding the cell header at the receiving end.